Analysis of the development of vasopressin and oxytocin neurons by immunocytochemical and RIA procedures have shown that there is a profound differential expression of peptides during fetal development. The vasopressin (AVP) neurons are more precocious than oxytocin (OT) neurons during development in extent of peptide expression and fiber outgrowth. This difference in peptide expression is due to an apparent absence of carboxypeptidase-B-like processing activity in the OT cells. The co-existence of AVP and CRF in nerve terminals in the external zone of the median eminence was conclusively demonstrated by serial section immunocytochemistry at the ultrastructural level, using antibodies against all three components of the AVP prohormone. Co-existence was found at the secretory vesicle level, implying co-secretion of both CRF and AVP from a specific subpopulation of nerve endings. Vasotocin (AVT) and mesotocin (MT) neurons have been mapped in 3-dimensions in adult Xenopus hypothalamus, and neurophysin immunoreactivities have been documented. Expression of these peptides appears at least at stage 45 in Xenopus development. An organotypic tissue culture model of rat LHRH neurons has been developed, and preliminary data suggests trophic interactions between brain stem, hypothalamus, and pituitary regions in vitro.